extension, and internal consistency (see the article by Pavlin and colleagues, elsewhere in this issue).

Choroidal Nevus

The Collaborative Ocular Melanoma Study (COMS) Group defined choroidal nevi as melanocytic lesions less than 5 mm in largest basal dimension and less than 1 mm in height.11 Ultrasonographic differentiation between choroidal nevi and small melanoma is difficult because of their small size. These lesions are often too flat to be detected by ultrasound. Suspicious nevi should be observed closely for changes in size. Nevi that are elevated enough for detection appear highly reflective and can be confused with other lesions with high reflectivity, such as choroidal hemangioma, metastatic carcinoma, and disciform lesions (Fig. 7).

Uveal Melanocytoma

Melanocytomas are benign dark brown or black tumors that predominantly involve the optic disc and the surrounding choroid and retina. Isolated melanocytoma of the iris, ciliary body, and choroid have been reported.12–14 These lesions are

elevated mildly and dome-shaped. They are typically highly reflective with regular internal structure and no internal vascularity (Fig. 8). Most melanocytomas remain stable,14–16 but subtle growth over several years is observed in approximately 10% of the cases.17,18 Malignant transformation into melanoma is reported in less than 2% of the cases.13–15,18–20

MALIGNANT UVEAL TUMORS

Iris and Ciliary Body Melanoma

Immersion techniques and high frequency ultrasound biomicroscopy are better suited than conventional contact ultrasonography for assessing iris and ciliary body melanomas because of their anterior location and smaller lesion size (See the article by Pavlin and colleagues, elsewhere in this issue).

Choroidal Melanoma

Choroidal melanomas exhibit various growth patterns. Small lesions typically appear as domeshaped, well-circumscribed, pigmented thickening of the choroids. As the lesions grow, the Bruch’s membrane may rupture to allow invasion

of the retina and extension into the vitreous cavity. If the Bruch’s membrane ruptures at the apex of the tumor, the lesion assumes a mushroom or collar button shape. When the Bruch’s membrane is ruptured at the rim of the tumor, it develops an irregular and inclined shape. Diffuse melanoma grows as an extensive thickening of the choroid and does not become elevated markedly. Regardless of their clinical presentation, all choroidal melanomas demonstrate characteristic ultrasonographic features (Box 1).

A-scan

Choroidal melanomas typically demonstrate low- to-medium internal reflectivity because of their homogeneous histologic architecture (Fig. 9). In larger lesions, sound attenuation is often observed with lower reflectivity at the base of the tumor. This is caused in part by the more homogeneous nature of the mass in this region. Moreover, lesions that contain hemorrhage, necrosis,

Box 1

Ultrasonographic features of uveal melanoma

A-scan

Low–medium reflectivity

Sound attenuation

Fast, spontaneous, low-amplitude flicker

B-scan

Collar button/dome shape

Solid consistency

Acoustic quiet zone

Choroidal excavation

Intrinsic vascular pulsations

and dilated vessels may show higher and more irregular internal reflectivity. Internal blood flow is another important acoustic property and can be appreciated by the presence of a fast, spontaneous, low-amplitude flickering within the internal tumor spikes.

B-scan

The pathognomonic appearance of choroidal melanoma is the collar button shape that results from a break in the Bruch’s membrane (Fig. 10). However, choroidal melanomas confined to the subretinal space, are dome-shaped, lobulated, and diffuse (see Fig. 9).

Choroidal melanoma appears on B-scan as an echo-dense lesion. The reflectivity is caused by internal acoustic interfaces between the cellular mass and varying degree of vascularity. At the tumor base, where the mass is more homogeneously cellular, and in relatively avascular lesions, an echolucent area can be seen. This area is called the acoustic quiet zone or acoustic hollowing.21

As the neoplasm infiltrates the normal surrounding choroid, it causes a bowl-shaped indentation at the margin of the tumor base. This feature is called choroidal excavation. It is not specific for melanoma and has been demonstrated in metastatic carcinoma.22 Bowing of the sclera posterior to the tumor has been reported in younger individuals and may indicate scleral infiltration.23 Exudative retinal detachment, subretinal hemorrhage, and vitreous hemorrhage often occur secondary to choroidal melanoma. Dense subretinal and/or vitreal hemorrhages potentially can mask the underlying tumor. In these cases, serial examinations must be performed to rule out choroidal melanoma and other tumors.

Extrascleral extension of choroidal melanoma appears as nodules near the base of the tumor.

These nodules are often echolucent because of sound attenuation from the primary mass. Congested blood vessels, extraocular muscle insertion, and inflammation in the sub-Tenon space can be mistaken for extrascleral growth (Fig. 11). These misinterpretations, however, may be avoided by serial examinations and being aware of the location and normal appearance of the extraocular muscle.

Ultrasonographic diagnosis of a diffuse melanoma can be challenging. Internal reflectivity is often difficult to assess because of its shallow nature. On B-scan, these tumors may have an irregular surface. Because of increased incidences of extrascleral extension,24 alternate imaging techniques such as MR I and fine needle aspiration biopsy should be considered in suspicious

The ultrasonographic differential